1. Field of the Invention
The present invention relates to a liquid ejection apparatus and a liquid agitation method, and more particularly, to a liquid ejection apparatus which ejects liquid toward a prescribed medium and a liquid agitation method for agitating the liquid.
2. Description of the Related Art
There is a liquid ejection apparatus which ejects dispersion liquid in which dispersed micro-particles are suspended. Examples of material of the micro-particles include, for instance, pigment, high-polymer resin, metal, glass, or oxide or compound of these. Generally, the micro-particles tend to aggregate and settle with the passage of time. When the liquid in which the micro-particles have aggregated and settled is ejected, then there is deterioration of quality in the ejection results, namely, density non-uniformities or distortions, poor color reproduction, non-uniform density of the micro-particles, and the like. Therefore, technology for agitating the dispersion liquid has been proposed.
Japanese Patent Application Publication No. 6-87220 discloses that piezoelectric elements are used as energy converting elements for ejecting ink, and are also driven to agitate the ink under a condition where ejection of the ink does not occur, thereby re-dispersing solids in the ink that have aggregated and settled.
Japanese Patent Application Publication No. 2002-96484 discloses that the piezoelectric elements are driven in a state where the nozzle openings are sealed with a sealing cap, thereby agitating pigment-based ink inside pressure chambers and a reservoir.
Japanese Patent Application Publication No. 2003-72104 discloses that a manifold for guiding ink to nozzles is provided with a first agitation device (piezoelectric element) to agitate the ink inside the manifold, a supply channel for supplying the ink to the manifold is provided with a second agitation device to agitate the ink inside the supply channel, and a container storing the ink is provided with a third agitation device to agitate the ink inside the container.
Japanese Patent Application Publication No. 2005-138488 discloses that an agitation mechanism for agitating ink inside an ink tank is provided, and a control unit for operating the agitation mechanism before supplying the ink from the ink tank is also provided.
In particular, in a liquid ejection apparatus having a liquid ejection head in which the liquid ejection face is situated in a bottommost position, nozzle blockages are liable to occur due to sedimented micro-particles in the nozzles. In the case of a so-called shuttle head structure in which the liquid ejection head performs a reciprocal back and forth movement, the liquid inside the liquid ejection head is agitated by the reciprocal motion of the liquid ejection head, but in the case of a line head structure where the liquid ejection head does not perform reciprocal movement, the liquid is not agitated usually.
Methods for agitating the liquid have been proposed, but it is difficult to agitate the liquid with good efficiency.
In Japanese Patent Application Publication Nos. 6-87220 and 2002-96484, the voltage of the input signal (drive signal) applied to the piezoelectric elements must be set within a range that does not cause liquid ejection, it is then impossible to vibrate the liquid with a relatively large amplitude, and hence a sufficient agitation effect cannot be obtained. More specifically, under the drive conditions in the range that does not cause the liquid ejection, the agitation performance inside the pressure chamber is poor and improvement in the state of micro-particles that have aggregated and sedimented in the vicinity of the nozzle openings cannot be expected.
Moreover, the liquid is present not only inside the liquid ejection head, but also in the tank storing the liquid supplied to the liquid ejection head, and in the liquid supply channel leading from the tank to the liquid ejection head. Since sedimentation and aggregation of the micro-particles occur at a plurality of locations, then, in order to achieve a uniform density of the liquid inside the whole apparatus, liquid agitation devices must be arranged at the plurality of locations.
For example, in Japanese Patent Application Publication No. 2003-72104, the liquid agitation devices are required in the manifold, the supply channels and the container; hence, not only does this raise costs, but it also leads to increase in the size of the apparatus.
In Japanese Patent Application Publication No. 2005-138488, in a case where a main tank and a sub-tank are provided as ink tanks, since the liquid is always present in both of the tanks, then it is necessary to provide the liquid agitation devices in both the main tank and the sub-tank.
However, in practice, it is difficult to provide liquid agitation devices in all of the locations in which the liquid is present separately within the apparatus.
Furthermore, in general, when the apparatus is left in a power-off state, the liquid ejection operation and the liquid supply operation are halted and no liquid agitation operation is carried out for a long period of time. In order to achieve a uniform liquid density within the whole apparatus when the power is turned on, it is necessary to agitate the liquid at different locations while circulating the liquid within the apparatus for a long duration, or the like, and accordingly it is necessary to lengthen the agitation operation duration. Hence, there is a problem in that the preparation duration before carrying out liquid ejection becomes long.